This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Operating Characteristics”.

1. How many factors are present in the operating characteristics?

a) 2

b) 3

c) 4

d) 5

View Answer

Explanation: There are 4 operating characteristics present in the single phase induction motor. They are mmf for air gap, saturation factor, iron loss, friction and windage loss.

2. How many parts does the stator mmf passes through?

a) 3

b) 4

c) 5

d) 6

View Answer

Explanation: The stator mmf produced in the motor passes through 5 parts. They are air gap, stator teeth, stator core, rotor teeth, rotor core.

3. What is the angle at which the value of the flux density should be for the calculation of mmf?

a) 40°

b) 60°

c) 80°

d) 70°

View Answer

Explanation: The calculation of mmf should be based upon the value of the flux density. The value of flux density at 60° from the interpolar axis as far as gap and teeth are concerned.

4. What is the value of the flux density with respect to average flux density?

a) value of flux density = 1.67 times of average flux density

b) value of flux density = 1.70 times of average flux density

c) value of flux density = 1.60 times of average flux density

d) value of flux density = 1.50 times of average flux density

View Answer

Explanation: The value of flux density at 60° from the interpolar axis as far as gap and teeth are used in the calculation of mmf. The value of flux density = 1.67 times of average flux density.

5. What is the formula for the mmf required for air gap?

a) mmf required for air gap = 8,00,000 * air gap flux density * air gap constant / air gap length

b) mmf required for air gap = 8,00,000 * air gap flux density * air gap constant * air gap length

c) mmf required for air gap = 8,00,000 * air gap flux density / air gap constant * air gap length

d) mmf required for air gap = 8,00,000 / air gap flux density * air gap constant * air gap length

View Answer

Explanation: The air gap flux density, air gap constant, air gap length are calculated first. On substitution, the mmf required for air gap can be obtained.

6. What is the formula for the saturation factor?

a) saturation factor = total mmf required for the magnetic circuit/mmf required for air gap

b) saturation factor = total mmf required for the magnetic circuit + mmf required for air gap

c) saturation factor = total mmf required for the magnetic circuit – mmf required for air gap

d) saturation factor = total mmf required for the magnetic circuit * mmf required for air gap

View Answer

Explanation: The total mmf required for the magnetic circuit and the mmf required for air gap is calculated. On substitution, the saturation factor is obtained.

7. What is the range of the saturation factor in the single phase induction motor?

a) 1.1-1.3

b) 1.0-1.2

c) 1.1-1.35

d) 1.2-1.6

View Answer

Explanation: The minimum value of the saturation factor in the single phase induction motor is 1.1. The maximum value of the saturation factor in the single phase induction motor is 1.35.

8. What is the relation between flux densities with respect to saturation factor?

a) flux density is indirectly proportional to the square of the saturation factor

b) flux density is directly proportional to the square of the saturation factor

c) flux density is indirectly proportional to the saturation factor

d) flux density is directly proportional to the saturation factor

View Answer

Explanation: The saturation factor is kept low if the flux densities in the teeth and core are low. The saturation factor is kept high if the flux densities in the teeth and core are high.

9. What factors are considered while calculating iron loss in stator teeth and core?

a) flux densities

b) weights

c) flux densities or weights

d) flux densities and weights

View Answer

Explanation: The iron loss in stator teeth and core are found by calculating their flux densities. The iron loss in stator teeth and core are found by calculating their weights.

10. What is the relation between total iron loss for induction motors and the sum of stator tooth and core loss?

a) total iron loss for induction motors = 1.3-2.3 times the sum of stator tooth and core loss

b) total iron loss for induction motors = 1.4-2.4 times the sum of stator tooth and core loss

c) total iron loss for induction motors = 1.5-2.5 times the sum of stator tooth and core loss

d) total iron loss for induction motors = 1.3-2 times the sum of stator tooth and core loss

View Answer

Explanation: The total iron loss for induction motors is 1.5-2.5 times the sum of stator tooth and core loss. The total iron loss is due to fundamental frequency flux.

11. What is the range of the multiplying factor when test data is not available?

a) 1.7-2

b) 1.75-2.2

c) 1.6-2.3

d) 1.5-2

View Answer

Explanation: The multiplying factor should be obtained from tests of motors of similar design. When test data is not available, a value of 1.75 to 2 may be used.

12. What are the factors which result in the bearing friction and windage loss?

a) ball bearings

b) sleeve bearing

c) ball bearing and sleeve bearing

d) ball bearing or sleeve bearing

View Answer

Explanation: The bearing friction and windage loss will depend upon the ball bearings. The bearing friction and windage loss will also depend upon the sleeve bearings.

13. What is the friction and windage loss for a 1500 rpm machine?

a) 3-7% of watt output

b) 3-9% of watt output

c) 4-8% of watt output

d) 1-5% of watt output

View Answer

Explanation: The friction and windage loss and a speed of 1500 rpm, it is usually minimum 4% of the watt output. The friction and windage loss and a speed of 1500 rpm, it is usually maximum of 8% of the watt output.

14. The high values actually apply for the small motors below 150 W.

a) true

b) false

View Answer

Explanation: The friction and windage loss and a speed of 1500 rpm, it is usually 4-8% of the watt output. The high values apply to small motors below 180 W.

15. The loss for the sleeve bearing having stator outer diameter 150 mm and 1000 rpm is 3.7 W.

a) true

b) false

View Answer

Explanation: The sleeve bearing having a stator outer diameter of 150 mm has losses at different running speed. The 1000 rpm machine gives loss of 3.7 W.

16. What is the range of the frequency constant?

a) 0.86-1.0

b) 0.82-1.0

c) 0.9-1.0

d) 0.5-1.0

View Answer

Explanation: The minimum value of the frequency constant used in the output equation of P.H Tricky is 0.86. The maximum value of the frequency constant used in the output equation of P.H Tricky is 1.0.

17. What is the range of the motor type constant?

a) 1.1-1.3

b) 1.0-1.4

c) 1.1-1.42

d) 1.1-1.5

View Answer

Explanation: The minimum value of the motor type constants is 1.1. The maximum value of the motor type constants is 1.42.

18. What is the formula of the most economical relation between D and L?

a) length = 0.6 * diameter

b) length = 0.5 / diameter

c) length = 0.6 / diameter

d) length = 0.63 * diameter

View Answer

Explanation: The length and the diameter are the 2 main dimensions of the single phase induction motor. The most economical relation between length and diameter is length = 0.63 * diameter.

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